[breaking change] Disambiguate extension type to declare an extension type

[eernstg]

Change Intent

The Dart grammar currently allows extension type on (int i,) {} as a declaration of an extension whose name is type and whose on-type is a record type.

With the upcoming introduction of extension type declarations, the language team wishes to ensure that it cannot be an extension, such that a reader of Dart code can safely assume that a declaration starting with extension type will be an extension type declaration.

The change proposed here is an adjustment of the grammar such that this goal is achieved. Details here: dart-lang/language#3431.

Justification

The introduction of extension type declarations creates an ambiguity, and it must be resolved. The language team prefers to ensure that a declaration that starts with extension type actually declares an extension type and not a plain extension, because that is considered to be the least surprising choice. Also, the value of being able to have an extension whose name is type is considered low.

Impact

Probably very low: It only affects extension declarations whose name is type, and that choice of name violates the general rule that type-like entities should have a capitalized name.

Mitigation

Rename the extension named type to some other name. This will affect explicitly resolved extension member invocations (where type must be replaced by the new name). It is expected to be a very rare phenomenon: Extensions named type are presumably very rare, and only few extension member invocations are resolved explicitly: Low probability times low probability is even lower probability.

Change Timeline

As soon as possible. This ambiguity will have to be handled at some point anyway.

Associated CLs

None, yet.

[leafpetersen]

cc @itsjustkevin @Hixie @vsmenon

I'd like to expedite review on this since it's on the critical path for release. This is a small technical breaking change which I don't expect to break anything in practice.

[itsjustkevin]

@grouma could you weigh in on this breaking change as well?

[vsmenon]

lgtm

[grouma]

SGTM.

[Hixie]

i strongly support this

[eernstg]

Very good, thanks!

The behavior has been implemented already, so there will not be a separate 'implementation' issue, I'll put the 'implementation' link here such that it is documented that the implementation has been handled.

[eernstg]

@sgrekhov, I believe we also have co19 tests for this topic, right?

[sgrekhov]

For extension we have
https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Extension-methods/syntax_t05.dart
https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Extension-methods/syntax_t06.dart

I'll add this corner case with record type as well